Sensorless motor braking system

ABSTRACT

An electric motor control system includes a power inverter and control circuitry configured to control the power inverter either according to a target voltage in a voltage-based control mode or according to a target current in a current-based control mode. A controller is operable to switch operation of the control circuitry between the voltage-based control mode and the current-based control mode. The controller may be configured to operate the control circuitry in the current-based control mode at lower motor operating speeds where stator current margin is of greater significance, and to operate the control circuitry in the voltage-based control mode at higher motor operating speeds where stator voltage margin is of greater significance.

RELATED APPLICATIONS

The present patent application is a divisional patent application whichclaims priority benefit with regard to all common subject matter to U.S.patent application Ser. No. 13/753,617, titled “SENSORLESS MOTOR BRAKINGSYSTEM”, filed Jan. 30, 2013. The earlier-filed patent application ishereby incorporated by reference into the current patent application inits entirety.

FIELD

Embodiments of the present invention relate to systems and methods forcontrolling electric motors.

BACKGROUND

Electric motors commonly include a stationary component called a statorand a rotating component called a rotor. The rotor rotates within (oraround) the stator when the motor is energized with a driving waveform.Induction motors, sometimes referred to as asynchronous motors, are atype of electric motor wherein power is supplied to the rotor by meansof electromagnetic induction rather than by means of direct electricalconnections to the rotor.

As with synchronous motors, the driving waveform supplied to aninduction motor's stator creates a magnetic field that rotates in timewith the AC oscillations of the driving waveform. The induction motor'srotor rotates at a slower speed than the stator field. This differencein rotational speed, also referred to as “slip,” “slip frequency,” or“slip speed,” results in a changing magnetic flux in the rotor windingsthat induces currents in the rotor windings. The induced currents inturn generate magnetic fields in the rotor windings that oppose therotating magnetic field created by the stator, thereby inducingrotational movement in the rotor. The rotor accelerates until themagnitude of induced rotor current and rotor torque balances the appliedload. Since rotation at synchronous speed would result in no inducedrotor current, an induction motor always operates at less thansynchronous speed during normal forward operation.

Rotation of an induction motor may be stopped by simply removing thedriving waveform from the motor and allowing the motor to coast to astandstill over time due to the inertia of the rotor and anythingcoupled to the rotor. Alternatively, rotation of an induction motor maybe stopped more quickly using a braking method involving adjusting thefrequency of the driving waveform to be less than the rotor frequency,wherein the rotating magnetic field created by the stator inducesrotational pressure on the rotor that opposes the rotor's movement andactively reduces motor speed. Using this braking method, the inertia ofthe rotor and applied load induces voltage in the stator that mayenergize external motor components, such as a DC bus supplying power tothe motor.

In many motor applications, it is desirable to stop rotation of therotor as soon as the driving waveform is removed from the motor. Forexample, in washing machine applications, it is desirable to stoprotation of the washing machine motor after a high speed spin cycle sothat the washing machine may be unloaded or switched to a slower speedwash or rinse cycle. Unfortunately, the braking method mentioned abovesuffers from limitations that may render it ineffective to quickly stopheavy loads. The braking pressure created by the driving waveform, forexample, is limited by the electric power available to energize thestator. The braking pressure is further limited by the capacity of themotor and other components to handle the voltage induced by the rotorduring the braking process.

Accordingly, various alternative techniques have been developed forbraking electric motors. One such technique uses brake pads, pulleys,and/or other friction braking systems. Unfortunately, friction brakesadd cost and complexity to a motor and are therefore not desirable forlow cost applications such as washing machines. Friction brakes alsoeventually wear out with use and require repair or replacement.

Thus, many motor applications employ alternative electric brakingsystems rather than friction brakes. One type of electric braking systeminvolves DC injection braking in which a direct current (DC) voltage isapplied to a motor's stator windings to brake the rotor. The DC voltagecreates a stationary magnetic field which applies a static torque to therotor. This slows and eventually halts rotation of the rotor. As long asthe DC voltage remains on the stator windings, the rotor is held inposition and resists rotation. DC injection braking is relativelysimple, cost-effective, and maintenance free and is therefore a popularchoice of braking for many motor applications; however, it has not beenused effectively in some applications as described below.

It is also often desirable to determine when a motor's rotor has stoppedrotating so the rotor can be driven in the opposite direction, at adifferent speed, etcetera. This can be accomplished with a motor shaftsensor such as a Hall effect sensor, but such sensors increase the costand complexity of motors and are therefore not desirable for many lowercost applications such as washing machine motors.

Thus, sensorless techniques for determining motor speed have beendeveloped. One type of sensorless speed detection employs variousalgorithms for estimating when a rotor stops based on measuredelectrical parameters. However, the measured electrical parameters, andthus the results of the algorithms, are less accurate when the motor isbeing braked with the above-described DC injection braking techniques.Thus, DC injection braking techniques generally require a motor shaftsensor.

The above section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

Embodiments of the present invention solve the above described problemsby providing improved systems and methods for braking electric motorswithout the need for motor shaft sensors, regenerative front-end diodes,braking resistors or other costly and/or complex components.

A braking mechanism constructed in accordance with an embodiment of thepresent invention and configured for use with an electric motor controlsystem utilizes a difference component and an integrating component togenerate a motor braking signal to augment braking power. The differencecomponent receives a first signal containing reference motor speedinformation, a second signal containing actual motor speed information,and generates a third signal containing information indicating adifference between the reference motor speed and the actual motor speed.The integrating component integrates the third signal and generates afourth signal based on the integrated third signal. The fourth signal isused to augment a motor braking signal generated by, for example, amotor control circuit that is part of the motor control system.

In another embodiment of the invention, a motor control system comprisesa power inverter and control circuitry configured to generate a firstmotor control signal according to a target voltage and to generate asecond control signal according to a target current. A first brakingmechanism is configured to modify the first control signal to maximizemotor losses, and a second braking mechanism is configured to modify thesecond control signal to maximize motor losses. A controller switchesoperation of the motor control system between generating the firstcontrol signal and the second control signal. The controller may alsoselectively engage either of the first braking mechanism and the secondbraking mechanism when the control circuitry is in a braking mode.

This summary is provided to introduce a selection of concepts in asimplified form that are further described in the detailed descriptionbelow. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Other aspectsand advantages of the present invention will be apparent from thefollowing detailed description of the embodiments and the accompanyingdrawing figures.

DRAWINGS

Embodiments of the present invention are described in detail below withreference to the attached drawing figures, wherein:

FIG. 1 is a block diagram of a motor system constructed in accordancewith embodiments of the invention.

FIG. 2 is a schematic representation of control circuitry for use withthe motor system of FIG. 1.

FIG. 3 is a schematic representation of an alternative embodiment of thecontrol circuitry of FIG. 2.

FIG. 4 is a graphical representation of certain operating andperformance characteristics of the motor of FIG. 1.

The drawing figures do not limit the present invention to the specificembodiments disclosed and described herein. The drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the invention.

DETAILED DESCRIPTION

The following detailed description of embodiments of the inventionreferences the accompanying drawings. The embodiments are intended todescribe aspects of the invention in sufficient detail to enable thoseskilled in the art to practice the invention. Other embodiments can beutilized and changes can be made without departing from the scope of theclaims. The following detailed description is, therefore, not to betaken in a limiting sense. The scope of the present invention is definedonly by the appended claims, along with the full scope of equivalents towhich such claims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or“embodiments” mean that the feature or features being referred to areincluded in at least one embodiment of the technology. Separatereferences to “one embodiment”, “an embodiment”, or “embodiments” inthis description do not necessarily refer to the same embodiment and arealso not mutually exclusive unless so stated and/or except as will bereadily apparent to those skilled in the art from the description. Forexample, a feature, structure, act, etcetera described in one embodimentmay also be included in other embodiments, but is not necessarilyincluded. Thus, the present technology can include a variety ofcombinations and/or integrations of the embodiments described herein.

Turning now to the drawing figures, and initially FIG. 1, an electricmotor system 10 constructed in accordance with embodiments of theinvention is illustrated. The motor system 10 broadly includes anelectric motor 12, a power source 14, and a motor control system 16 forreceiving power from the power source 14 and generating a drivingwaveform to power the motor 12 according to a reference motor speed(ω_(ref)) communicated to the control system via a reference speed input18. The reference motor speed corresponds to a desired or targetoperating motor output speed and may be generated by an external motorcontrol mechanism (not illustrated) configured to operate the motor 12according to a particular implementation of the invention. The motorcontrol system 16 includes control circuitry 20 configured to receiveinput power from the power source 14, receive a reference motor speedsignal from the reference speed input 18, create the driving waveform,and communicate the driving waveform to the motor 12. A controller 22manages certain aspects of the operation of the control circuitry 20,including switching between a voltage-based control mode and acurrent-based control mode, as explained below.

The power source 14 may be a conventional alternating-current powersource, such as a standard 115V or 230V source available in residentialand commercial buildings via standard electrical outlets. The motorsystem 10 may be part of an appliance drive system, such as a washingmachine or a clothes dryer, that includes additional electrical andmechanical components not illustrated or described herein.

The motor 12 may be an electric induction motor and, in one exemplaryembodiment, is a three-phase, four-pole AC induction motor rated tooperate at a maximum voltage of between 190 and 200 volts and a maximumcurrent of between 4 and 6 amps. The motor 12 may be configured tooperate within a normal operating range of, for example, between 0 and3,000 rpm, and may be capable of operating at speeds well beyond thenormal operation range, such as up to 17,000 or 18,000 rpm. Motoroperation beyond the normal operating range may be characterized byprogressively diminishing performance characteristics, such as decreasedtorque, as explained below.

Various operating and performance characteristics of the motor 12,including stator voltage (V_(s)), maximum operating current and outputtorque, are illustrated in the graph of FIG. 4 across the operatingspeed range of the motor 12. As shown in FIG. 4, motor performancecharacteristics differ according to whether the motor is operating in anormal operating region or a field weakening region. The normaloperating region corresponds to a motor output speed of zero up to anominal operating speed 24, which may be, for example, 3,000 or 4,000rpm. The field weakening operating region corresponds to the nominalspeed 24 up to a maximum motor speed 26, which may be, for example,about 17,000 or 18,000 rpm. When operating in the normal operatingregion, the speed of the motor 12 generally follows a linear pattern andis proportional to the stator voltage (V_(s)), while torque is generallyconstant. In the field weakening region, the stator voltage remains ator near a maximum operating voltage and the torque diminishes as themotor output speed increases. The maximum operating current is definedby limitations of the motor 12, the control circuitry 20, or both. Whilethe maximum operating current generally remains constant across allmotor operating speeds, the motor stator current (i_(s)) will typicallyfluctuate with changing operational parameters, such as changes in motorspeed and load torque. The total stator current (i_(s)) includes atorque producing component (i_(sq)) and a flux producing component(i_(sd)), whose relationship is defined by the following equation:i _(s)=√{square root over ((i _(sd))²+(i _(sq))²)}

For purposes of the present teachings, the motor 12 generally operatesin three different output speed ranges including low speed, medium speedand high speed. The low speed range generally corresponds to the normaloperating region, the medium speed range generally corresponds to alower portion of the field weakening region, and the high speed rangegeneral corresponds to an upper portion of the field weakening region,as indicated in the graph of FIG. 4. The motor speed corresponding tothe boundary between the medium and high speed ranges may be defined asa particular motor speed, such as 8,000 rpm or 10,000 rpm, or as apercentage of a maximum motor speed, such as 50%, 60% or 70% of themaximum speed 26. Alternatively, the boundary may be defined in terms ofthe stator current, such as a particular value of i_(sd) or i_(sq) or adifference between i_(sd) and i_(sq). Further yet, the boundary betweenthe medium and high speed ranges may be defined in terms of the torque,such as a particular torque value or percentage of maximum torque. Theboundary between the medium and high speed ranges is relevant in thatmay be used by the controller as a threshold speed for switching betweenthe voltage-based control mode and the current-based control mode, asexplained below. In some embodiments, the boundary between the mediumand high speed ranges is defined as a rotor speed at which the statorvoltage amplitude is at or near the inverter voltage margin. When themotor is operating at or near the inverter voltage margin, fluctuationsin the inverter voltage margin due to, for example, variations in the DCbus voltage make current-based motor control impractical.

The controller 22 includes digital logic configured to interact with andcontrol certain components of the control circuitry 20. The digitallogic may include discrete logic components, programmable logic devicesand/or general purpose computer processors such as microcontrollers ormicroprocessors. In some embodiments of the invention, the controller 22is or includes a computer processor configured to execute a computerprogram for directing the control circuitry 20 to operate in either thecurrent-based control mode in the low and medium speed ranges or in thevoltage-based control mode in the high speed range.

Thus, aspects of the invention may be implemented with one or morecomputer programs stored in or on computer-readable media residing in oraccessible by the controller 22. The computer program comprises a seriesof executable instructions for implementing logical functions in thecontroller 22. The computer program can be embodied in anynon-transitory computer-readable medium for use by or in connection withthe controller 22 wherein the controller fetches instructions from thecomputer-readable medium and executes the instructions. As used herein,a “computer-readable medium” can be any non-transitory means that canstore the program for use by or in connection with the controller. Thecomputer-readable medium can be, for example, but is not limited to, anelectronic, magnetic, optical, electro-magnetic, infrared, orsemi-conductor system, apparatus, or device. More specific, although notinclusive, examples of the computer-readable medium would include thefollowing: an electrical connection having one or more wires, a randomaccess memory (RAM), a read-only memory (ROM), and an erasable,programmable, read-only memory (EPROM or Flash memory).

Turning now to FIG. 2, certain components of the control circuitry 20are illustrated in greater detail. A rectifier 30 converts AC powerreceived from the power source 14 to DC power that energizes a DC bus32. The rectifier 30 may include virtually any type of rectifiercircuit, but in various preferred embodiments is a simplenon-regenerative rectifier, such as a diode bridge rectifier. A signalshunt 34 and voltage sensor 35 are coupled with the DC bus 32 and usedto detect voltage and/or current levels on the DC bus 32 for use infeedback loops, as explained below in greater detail. The signal shunt34 may be, for example, a resister, while the voltage sensor 35 may be avoltage divider circuit. One or more capacitors 36 are also coupled tothe DC bus 32 to minimize voltage ripple on the bus 32 and to storeregenerative energy in the bus 32 during braking.

Inverter circuitry 38 includes pulse width modulation (“PWM”) controllogic 40 and a three-phase inverter 42 that converts DC power from theDC bus 32 to driving waveforms that are communicated to the motor 12.The PWM control logic 40 receives two input signals (V_(αref) andV_(βref)) representing two-phase stationary reference frame voltagevalues V_(α) and V_(β), and converts the input signals to invertercontrol signals that drive the inverter to convert power from the DC bus32 to the driving waveforms applied to the motor stator windings. Thevoltage magnitude, voltage angle and frequency of the driving electricwaveforms are manipulated according to the reference motor speed ω_(ref)to control operation of the motor 12. When the motor 12 is operating ata constant speed or accelerating, it has a positive slip frequency andgenerally consumes energy. When the motor 12 is decelerating (braking),it has a negative slip frequency and generates energy that is dissipatedin the motor 12 (as rotor and stator losses) and that energizes theinverter 42 and the DC bus 32, thereby increasing the voltage in thosecomponents beyond the voltage supplied by the rectifier 30. Theincreased voltage on the DC bus 32 may contribute to braking by furtherincreasing stator voltages and losses in the motor 12, as explainedbelow in greater detail.

When the switch 28 is in a first position (represented by the brokenlines in FIG. 2), the control circuitry 20 operates in the current-basedcontrol mode according to principles of indirect field oriented control(IFOC). In the current-based control mode, the control circuitry 20drives operation of the power inverter 42 according to a target statorcurrent. The IFOC motor control method involves controlling AC motoroperation by means of three motor input variables: voltage magnitude,voltage angle and signal frequency. These variables are manipulatedaccording to the reference speed ω_(ref) via two control/feedback loopsthat independently control the speed (frequency) and strength of therotating magnetic field induced in the motor's stator.

The reference motor speed (ω_(ref)) signal is received at the left sideof the circuit diagram in FIG. 1. The ω_(ref) signal may be receivedfrom an external motor controller, such as an appliance motor controllerconfigured to control the speed and operation of the motor according tooperational parameters of the particular appliance. An estimated rotorspeed, ω_(r), is subtracted from the reference speed and the differenceis communicated to a first proportional-integral controller 44configured to generate a torque-producing component i_(qref) of a targetstator current. The estimated motor speed ω_(r)) is also communicated toa field weakening controller 46 configured to generate a flux-producingcomponent i_(dref) of the stator current. The field weakening controller46 decreases the value of i_(dref) when the reference motor speed is inthe field weakening region of motor operation.

The i_(qref) signal is communicated from the first proportional-integralcontroller 44 to a DC voltage limiter circuit 48 that is configured tolimit the maximum negative value of i_(qref) based on the voltagelimitations of the motor 12, inverter 42 and/or capacitor 36. The valueof i_(qref) is negative during motor braking when the reference motorspeed is less than the actual rotor speed. When braking, regenerativeenergy from the motor 12 increases the voltage in the inverter 42 andthe DC bus capacitor 36, as explained above. The voltage limiter circuit48 prevents the negative value of i_(qref) from exceeding a levelcorresponding to the maximum voltage of the motor, inverter and/orcapacitor. The voltage limiter circuit 48 receives a first signalV_(dcmax) representing a maximum voltage level, a second signal V_(dc)representing an estimated or measured voltage on the DC bus 32, andsubtracts the value of V_(dc) from V_(dcmax). An integrator controller50 integrates the difference and communicates the result, −i_(qrefmax),to a first current limiter component 52 that limits i_(qref) to thevalue of −i_(qrefmax).

A current limiter component 54 receives both i_(qref) and i_(dref)signals and imposes a limit on the signals corresponding to a maximumstator current as defined by a maximum rating of the motor 12 and/or theinverter 42. Thus, while the voltage limiter circuit 48 imposes amaximum negative value of i_(qref) during braking to regulateregenerative voltage, the current limiter component 54 imposes a maximumpositive value of i_(qref) and i_(dref) during normal operation.Measured values of the torque-producing component i_(q) and theflux-producing component i_(s) of the actual stator current aresubtracted from i_(qref) and i_(dref), and the resulting difference iscommunicated to second 56 and third 58 proportional-integral controllerswhich calculate rotating two-phase voltage signals V_(qref) andV_(dref).

An inverse Park conversion circuit 60 converts the rotating V_(qref) andV_(dref) signals to stationary two-phase signals V_(αref) and V_(βref)which are received by the PWM control logic 40 of the inverter circuitry38. The PWM control logic 40 generates a plurality of inverter controlsignals based on the stationary two-phase signals V_(αref) and V_(βref),as explained above, and communicates the control signals to the inverter42.

In the current-based control mode, the control circuitry 20 usesfeedback loops to generate the motor control signals. The shunt 34 andsensor 35 detect current and/or voltage on the DC bus 32 and communicatethe information to a converter component 62 that generates a pluralityof signals representing measured values of the instantaneous statorcurrent and/or voltage. A Clarke conversion circuit 64 converts themeasured stator current and/or voltage values to two-phase, stationaryvalues i_(α) and i_(β), which are then converted to two-phase rotatingsignals i_(q) and i_(d) by a Park conversion circuit 66. The i_(q) andi_(d) signals are combined with the i_(qref) and i_(dref) signals, asexplained above. The Clarke conversion circuit 64 may also generatetwo-phase, stationary values V_(α) and V_(β) that are used by a speedestimator 68 to estimate the rotor speed ω_(r).

The speed estimator 68 calculates an estimated rotor speed, ω_(r), basedon characteristics of the power communicated to the stator, such as theestimated values i_(α), i_(β), V_(α) and V_(β) generated by the Clarkeconversion circuit 64. The speed estimator 68 uses, for example, modelreference adaptive system (MRAS) principles of operation. The ω_(r)signal is combined with ω_(ref) to generate i_(qref), as explainedabove. The ω_(r) signal is also used by an output angle calculationcircuit 70 to generate an output angle signal θ_(dq) that is used by theClarke and inverse Clarke conversion circuits to translate between thestationary and rotating domains.

The output angle calculation circuit 70 includes a slip calculator 72that determines slip frequency, ω_(s), as a function of i_(qref) andi_(dref). Specifically, the slip frequency may be defined using thefollowing equation:

$\omega_{s} = {\frac{i_{qref}}{i_{dref}} \times k}$where the value of k depends on the characteristics of the motor 12. Theslip frequency is combined with the estimated rotor frequency, ω_(r),and that combination is integrated using an integrator 74 to generatethe output angle signal θ_(dq).

As mentioned above, when operating in the current-based control mode thecontrol circuitry 20 uses two independent feedback loops to control thestrength and speed of the rotating magnetic field induced in the stator.A first feedback loop controls the strength of the rotating magneticfield and includes the Park 66 and Clarke 64 conversion circuits and theproportional-integral controllers 56, 58. A second feedback loopcontrols the speed of the rotating magnetic field and includes theClarke conversion circuit 64, the speed estimator 68, the firstproportional-integral controller 44, the field weakening controller 46and the output angle calculation circuit 70. Thus, there is some overlapbetween the two feedback loops.

When the switch 28 is in the second position (indicated by solid linesin FIG. 2) the control circuitry 20 operates in the voltage-basedcontrol mode and controls the motor 12 by manipulating the voltage andfrequency of the driving waveform rather than using the IFOC mode ofoperation employed in the current-based control mode. In the illustratedembodiment, V_(dref) is set to zero and V_(qref) is set to V_(rated),which may correspond to the maximum operating voltage. The maximumoperating voltage will typically be the voltage available on the DC bus32, but may also be related to the maximum voltage capacity of theinverter 42 and/or the motor 12.

In the voltage-based control mode, the control circuitry 20 operates theinverter 42 according to a target voltage (V_(rated)) rather than atarget current. This mode of operation may be desirable, for example,where the motor 12 is operating at very high speeds and stator voltageis at or near the maximum voltage. In such situations, the voltage-basedcontrol mode allows greater power to be delivered to the motor 12because the voltage margins associated the current-based control modeare eliminated.

Control circuitry 100 constructed according to another embodiment of theinvention is illustrated in FIG. 3. The control circuitry 100 isidentical to the control circuitry 20 illustrated in FIG. 2 anddescribed above, except that the control circuitry 100 includes brakemechanisms for regulating the braking power of the motor 12 when thecontrol circuitry 100 is operating in either the current-based controlmode or in the voltage-based control mode. Therefore, for conveniencesimilar components are assigned the same reference numerals and only thedifferences between the two embodiments will be discussed. Particularly,a current-based braking mechanism 102 is used when the circuitry 100 isoperating in the current-based control mode and a voltage-based brakingmechanism 104 is used when the circuitry 100 is operating in thevoltage-based control mode.

The current-based brake mechanism 102 determines a difference between achange in the reference speed (Δω_(ref)) and a change in the estimatedrotor speed (Δω_(r)). That difference is integrated using an integratorcontroller 106 to generate a differential speed signal Δi_(dref) that iscombined with the i_(dref) signal when braking in the current-basedcontrol mode. Thus, the current-based braking mechanism 102 augments thei_(dref) signal according to a difference between the reference motorspeed ω_(ref) and the estimated rotor speed ω_(r) thereby increasingmotor losses and causing the motor to slow at a quicker rate. As thedifference between the reference motor speed and the estimated motorspeed increases, so does the value of Δi_(dref) and i_(dref).

A first braking switch 108 is operable to selectively connect the outputof the current-based braking mechanism 102 to the i_(dref) signalcombination point. The controller 22 may be configured to actuate theswitch 108 to connect the current-based braking mechanism 102 to thei_(dref) signal combination point when the motor is in braking mode(i.e., when ω_(ref) is less than ω_(r)).

The voltage-based braking mechanism 104 augments the V_(rated) input toincrease motor losses during braking when the control circuitry 20 isoperating in the voltage-based control mode. The voltage-based brakingmechanism 104 is similar in form and function to the current-basedbraking mechanism 102, described above, such that the braking signalincreases with a difference between the reference speed and theestimated rotor speed. Augmenting the V_(rated) input causes the voltageapplied to the motor to increase as well, thus increasing motor lossesduring braking and causing the motor to slow at a quicker rate.Increasing the voltage applied to the motor also increases the level ofenergy generated in the inverter and on the DC bus by the motor, whichin turn increases the amount of voltage available to be applied to themotor, further increasing motor losses and braking power.

A second braking switch 110 is operable to selectively connect theoutput of the voltage-based braking mechanism 104 to the V_(rated)signal combination point. The controller 22 may be configured to actuatethe switch 110 to connect the voltage-based braking mechanism 104 to theV_(rated) signal combination point when the motor is in braking mode(i.e., when ω_(ref) is less than ω_(r)).

As explained above, the amount of energy generated in the inverter 42and the DC bus 32 by the motor 12 during braking will generally belimited by the rated capacity of the motor 12, the inverter 42 and/orthe capacitor 36. When braking in either the voltage-based control modeor the current-based control mode, voltage generated on the DC bus 32 bythe motor 12 is limited by the voltage limiting circuit 48. In thecurrent-based control mode, the voltage limiting circuit 48 regulatesboth i_(qref) and the motor slip frequency to limit energy generation inthe inverter 42 and on the DC bus 32 and thus avoid damage to thosecomponents. In the voltage-based control mode, the voltage limitingcircuit 48 regulates only the slip frequency.

It will be appreciated by those skilled in the art that the brakingmechanisms 102, 104 provide enhanced braking without compromisingcontrol of the braking process. The speed of the motor 12, for example,is tracked by the speed estimator 68 such that braking is enabled todecrease motor speed to any speed, including speeds other than zero. Itmay be desirable, for example, to slow the motor speed from 5,000 rpm to3,000 rpm.

In operation, the controller 22 actuates the circuit control switch 28to shift operation of the control circuit between the voltage-basedcontrol mode and the current-based control mode. The controller 22further actuates the first brake switch 108 and the second brake switch110 to use the brake mechanisms 102, 104 when in braking mode. Operationof the motor system 10 typically begins when the actual speed of themotor is zero and some non-zero reference speed is applied to the motorcontrol system 16. The controller 22 actuates the circuit control switch28 such that the switch 28 is in the first position to place the controlcircuitry in the current-based control mode. The control circuitry 20generates a driving waveform causing the motor speed to graduallyincrease toward the motor reference speed. As the motor speed increasesthrough the normal operating region, the stator voltage increaseslinearly with the speed of the motor 12, as illustrated in FIG. 4. Ifthe reference motor speed ref is in the normal operating region, theactual motor speed reaches the reference speed and stabilizes, and thecontrol circuit 20 continues to drive the motor 12 at the referencespeed in the current-based control mode.

If the reference motor speed ω_(ref) is in the medium speed range, thecontrol circuitry 20 causes the motor speed to increase such that thestator voltage increases to and remains near the maximum availablevoltage as the actual motor speed reaches the nominal operating speedthreshold. The motor speed continues to increase in the field weakeningregion as the torque-producing component of the stator currentincreases, the flux-producing component of the stator current decreases,and the motor torque decreases. The actual motor speed eventuallyreaches the reference speed and stabilizes, and the control circuit 20continues to drive the motor 12 at the reference speed in thecurrent-based control mode.

If the reference speed is in the high speed range of FIG. 1, thecontroller 22 continues to increase the motor speed in the current-basedcontrol mode until the estimated motor speed Wr reaches the high speedrange, wherein the controller 22 drives the switch 28 from the firstposition to the second position, shifting the control circuitry 20 fromthe current-based control mode to the voltage-based control mode. In thevoltage-based control mode, the control circuitry 20 operates the motor12 at the maximum rated voltage, as explained above. As long as thereference motor speed remains in the high speed range, the controlcircuitry 20 continues to operate in the voltage-based control mode.

While the motor 12 is operating in the high speed range, if thereference motor speed decreases the control circuitry 20 adjustsi_(qref) and i_(dref) causing the motor to operate at a negative slipfrequency until ω_(r) decreases to correspond to ω_(ref). Operating at anegative slip frequency uses the driving waveform against the motor'smovement and therefore acts as a brake. In this braking mode, themotor's movement generates electrical energy that may drive up thevoltage in the inverter 42 and/or the DC bus 32. If the voltage in theinverter 42 and/or the DC bus 32 approaches the maximum voltageV_(dcmax), the voltage limiting circuit 48 responds by limiting thenegative slip frequency to thereby limit the V_(dc) value and preventdamaging overvoltage.

If the control circuitry 20 includes the voltage-based braking mechanism104, as soon as the reference motor speed decreases to a level below theestimated speed ω_(r), the controller 22 drives the second brake switch110 to combine the voltage value V_(rated) with V_(qref) and therebyincrease braking power. When the actual motor speed approaches orreaches the reference motor speed, the controller 22 drives the secondbrake switch 110 to disengage the braking mechanism 104 from the controlcircuitry 20.

As long as the reference speed is within the high speed range, thecontrol circuitry 20 continues to operate in the voltage-based controlmode. If the reference speed decreases from the high speed range to themiddle or low speed range, however, the controller 22 switches operationof the control circuitry 20 from the voltage-based control mode to thecurrent-based control mode by driving the switch 28 from the secondposition to the first position. If the motor system 10 brakes while inthe current-based control mode, the controller 22 drives the first brakeswitch 108 to engage the current-based braking mechanism 102 asexplained above, thereby augmenting the i_(dref) signal and increasingbraking power.

The advantages of the present invention will be apparent to thoseskilled in the art. The design of the motor system 10 minimizes costs inthat it does not require the use of a braking resistor, a regenerativefront-end diode bridge, or motor shaft speed sensors. The system 10operates at maximum efficiency by operating according to a targetcurrent at lower speeds where the current margin governs operation, andoperating according to a target voltage at higher speeds where thevoltage margin governs operation. Furthermore, the motor system 10includes an enhanced braking system that can quickly brake the motor toany speed without losing the ability to measure the motor's speed.

Although the invention has been described with reference to thepreferred embodiment illustrated in the attached drawing figures, it isnoted that equivalents may be employed and substitutions made hereinwithout departing from the scope of the invention as recited in theclaims. The motor control system may be powered by a direct currentsource, for example, rather than the alternating current source asillustrated, wherein the rectifier may be omitted. Furthermore, thestator current and voltage values used in the control loops may bederived directly from motor input conductors rather than from the DCbus.

Having thus described the preferred embodiment of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:
 1. A motor control system comprising: control circuitryconfigured to generate a first control signal according to a targetvoltage and to generate a second control signal according to a targetcurrent; a first braking mechanism for modifying the first controlsignal to maximize motor losses, the first braking mechanism configuredto augment a maximum voltage value of the first control signal, thefirst braking mechanism including a difference component for receiving afirst signal containing motor reference speed information, for receivinga second signal containing actual motor speed information, and forgenerating a third signal containing information indicating a differencebetween the reference speed and the actual speed, and an integratingcomponent for integrating the third signal and communicating anintegrated signal to the control circuitry; a second braking mechanismfor modifying the second control signal to maximize motor losses; and acontroller for switching operation of the motor control system betweengenerating the first control signal and the second control signal. 2.The motor control system of claim 1, further comprising a first switchconfigured to selectively engage the first braking mechanism, thecontroller being operable to activate the first switch to engage thefirst braking mechanism when the motor control system is operating togenerate the first control signal and is operating in a braking mode. 3.The motor control system of claim 1, the second braking mechanismconfigured to augment a flux producing current component of the secondcontrol signal.
 4. The motor control system of claim 3, the secondbraking mechanism including: a difference component for receiving afirst signal containing motor reference speed information, for receivinga second signal containing actual motor speed information, and forgenerating a third signal containing information indicating a differencebetween the reference speed and the actual speed, and an integratingcomponent for integrating the third signal and communicating anintegrated signal to the control circuitry.
 5. The motor control systemof claim 4, further comprising a second switch configured to selectivelyengage the second braking mechanism, the controller being operable toactivate the second switch to engage the second braking mechanism whenthe motor control system is operating to generate the second controlsignal and is operating in a braking mode.
 6. The motor control systemof claim 1, the control circuitry including: a DC bus for deliveringpower to a power inverter at a forward operating power level, and acapacitor coupled with the DC bus and operable to receive and storeelectrical energy generated that exceeds the forward operating powerlevel.
 7. The motor control system of claim 6, further comprising aregulator for limiting an amount of voltage induced by the motor duringbraking by regulating a slip frequency of the motor.
 8. A motor controlsystem comprising: control circuitry configured to generate a firstcontrol signal according to a target voltage and to generate a secondcontrol signal according to a target current; a first braking mechanismincluding a difference component for receiving a first signal containingmotor reference speed information, for receiving a second signalcontaining actual motor speed information, and for generating a thirdsignal containing information indicating a difference between thereference speed and the actual speed, and an integrating component forintegrating the third signal and communicating an integrated signal tothe control circuitry; a second braking mechanism including a differencecomponent for receiving a fourth signal containing motor reference speedinformation, for receiving a fifth signal containing actual motor speedinformation, and for generating a sixth signal containing informationindicating a difference between the reference speed and the actualspeed, and an integrating component for integrating the sixth signal andcommunicating an integrated signal to the control circuitry; and acontroller for switching operation of the motor control system betweengenerating the first control signal and the second control signal. 9.The motor control system of claim 8, the first braking mechanismconfigured to augment a maximum voltage value of the first controlsignal.
 10. The motor control system of claim 8, further comprising afirst switch configured to selectively engage the first brakingmechanism, the controller being operable to activate the first switch toengage the first braking mechanism when the motor control system isoperating to generate the first control signal and is operating in abraking mode.
 11. The motor control system of claim 8, the secondbraking mechanism configured to augment a flux producing currentcomponent of the second control signal.
 12. The motor control system ofclaim 8, further comprising a second switch configured to selectivelyengage the second braking mechanism, the controller being operable toactivate the second switch to engage the second braking mechanism whenthe motor control system is operating to generate the second controlsignal and is operating in a braking mode.
 13. The motor control systemof claim 8, the control circuitry including: a DC bus for deliveringpower to a power inverter at a forward operating power level, and acapacitor coupled with the DC bus and operable to receive and storeelectrical energy generated that exceeds the forward operating powerlevel.
 14. The motor control system of claim 13, further comprising aregulator for limiting an amount of voltage induced by the motor duringbraking by regulating a slip frequency of the motor.
 15. A motor controlsystem comprising: control circuitry configured to generate a firstcontrol signal according to a target voltage and to generate a secondcontrol signal according to a target current; a first braking mechanismconfigured to augment a maximum voltage value of the first controlsignal, the first braking mechanism including a difference component forreceiving a first signal containing motor reference speed information,for receiving a second signal containing actual motor speed information,and for generating a third signal containing information indicating adifference between the reference speed and the actual speed, and anintegrating component for integrating the third signal and communicatingan integrated signal to the control circuitry; a second brakingmechanism configured to augment a flux producing current component ofthe second control signal, the second braking mechanism including adifference component for receiving a fourth signal containing motorreference speed information, for receiving a fifth signal containingactual motor speed information, and for generating a sixth signalcontaining information indicating a difference between the referencespeed and the actual speed, and an integrating component for integratingthe sixth signal and communicating an integrated signal to the controlcircuitry; and a controller for switching operation of the motor controlsystem between generating the first control signal and the secondcontrol signal.
 16. The motor control system of claim 15, furthercomprising: a first switch configured to selectively engage the firstbraking mechanism, the controller being operable to activate the firstswitch to engage the first braking mechanism when the motor controlsystem is operating to generate the first control signal and isoperating in a braking mode, and a second switch configured toselectively engage the second braking mechanism, the controller beingoperable to activate the second switch to engage the second brakingmechanism when the motor control system is operating to generate thesecond control signal and is operating in a braking mode.
 17. The motorcontrol system of claim 15, the control circuitry including: a DC busfor delivering power to a power inverter at a forward operating powerlevel, and a capacitor coupled with the DC bus and operable to receiveand store electrical energy generated that exceeds the forward operatingpower level.
 18. The motor control system of claim 17, furthercomprising a regulator for limiting an amount of voltage induced by themotor during braking by regulating a slip frequency of the motor.
 19. Amotor control system comprising: control circuitry configured togenerate a first control signal according to a target voltage and togenerate a second control signal according to a target current; a firstbraking mechanism for modifying the first control signal to maximizemotor losses; a second braking mechanism for modifying the secondcontrol signal to maximize motor losses, the second braking mechanismconfigured to augment a flux producing current component of the secondcontrol signal, the second braking mechanism including a differencecomponent for receiving a first signal containing motor reference speedinformation, for receiving a second signal containing actual motor speedinformation, and for generating a third signal containing informationindicating a difference between the reference speed and the actualspeed, and an integrating component for integrating the third signal andcommunicating an integrated signal to the control circuitry; and acontroller for switching operation of the motor control system betweengenerating the first control signal and the second control signal. 20.The motor control system of claim 19, further comprising a second switchconfigured to selectively engage the second braking mechanism, thecontroller being operable to activate the second switch to engage thesecond braking mechanism when the motor control system is operating togenerate the second control signal and is operating in a braking mode.